As one example of an integrated circuit (IC) that begins operation from an initial condition after the application of a power signal, consider a dynamic random access memory (DRAM) formed on a substrate. Such an IC conventionally accepts an externally applied power signal (V.sub.CCX) on one of its contacts. To operate, V.sub.CCX is applied with a voltage in a range including 5 volts measured relative to a ground contact. For retaining data stored in the memory while it operates, V.sub.CCX is supplied from a battery in a range including 3.3 volts. Operation from an initial condition follows, for example, two different types of transitions: the transition from 0 volts to 5 volts for beginning operation of the DRAM, and the transition from 3.3 volts to 5 volts for resuming operation of a DRAM containing previously stored data.
Whether beginning with or without previously stored data, circuits within the DRAM may not correspond to the initial condition until after receiving an initialization signal. Conventional initialization signals include a pulse having a leading edge generated when a change in the applied power signal is detected, and a trailing edge generated when a predetermined time period has lapsed. The conventional timer circuit used for generation of the initialization signal does not operate properly when the voltage of the applied power signal changes too slowly.
Improper operation is most apparent when the timer circuit used to generate the initialization pulse includes a current source driven from V.sub.CCX. The current source provides a regulated current for charging a capacitor of substantial capacitance. When the voltage developed on the capacitor exceeds a threshold, the trailing edge of the initialization pulse is generated. But, when the voltage of V.sub.CCX rises slowly over a considerable time, for example 6 milliseconds per volt, the voltage on the capacitor may exceed the threshold before V.sub.CCX is of sufficient magnitude to achieve reliable initialization.
Proper operation depends on reliable initialization, even when the applied V.sub.CCX signal cannot be generated with a sufficiently rapid transition to the operating level. When an integrated circuit such as a DRAM is not properly initialized, i.e. does not begin from a predetermined initial condition, inaccurate data may be recalled or new data may not be accurately stored. System failure is likely to result from use of such unreliably stored data.
Memory devices are commonly used in a wide variety of system designs, including computer, telecommunications, banking, video, and audio equipment, to name a few major applications. When system performance requires additional memory capability, such as proper operation in spite of slowly varying power supply voltage, it is desirable to deliver such additional capability without adversely affecting other aspects of the system design. In other words, it is desirable for improved memory to be compatible with existing system designs so that expensive redesign can be avoided.
In view of the problems described above and related problems that consequently become apparent to those skilled in the applicable arts, the need remains in integrated circuit technology for a power up initialization circuit that, among other capabilities, operates reliably when the applied power signal is characterized by a slow transition to an operating power level.